[Cashtab, ecash-herald] Add ecash-wallet as dep in dockerfiles as it is now needed to build ecash-agora

Summary: Needed now that ecash-agora uses ecash-wallet as a dev dep to clear current cashtab CI error

Test Plan:

docker build -f cashtab.Dockerfile -t cashtab_local .
docker build -f ecash-herald.Dockerfile -t ecash-herald_local .

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18687